A number of process units, such as evaporators, operated in series to perform a particular duty. In the case of multiple-effect evaporators, each successive stage has a greater vacuum and operates at correspondingly lower temperature. This enables the heat in the vapour leaving each successive effect to be used to heat the next. There are several arrangements that are commonly used, including feed forward in which the product and vapour both move on to subsequent effects. In backward feed operations, the vapour feeds in the reverse direction starting in the last effect. This requires pumping to overcome the higher pressures. The heating of the feed in the last effect, which has the lowest temperature, requires a lower temperature difference for evaporation. In parallel feed operation, the vapour feeds from one effect to another while fresh feed is fed to each effect independently.